The invention relates to a method of performing contention resolution for dedicated channel reservation of a cellular radio network air interface, which comprises: a network part and a radio connection from the network part to at least one subscriber terminal; the network part captures a random access request message transmitted on a random access channel by the subscriber terminal; the network part transmits an access grant message on an access grant channel to the subscriber terminal; at least one subscriber terminal receives the access grant message on the access grant channel and on the basis thereof assumes having reserved a dedicated channel; a network layer of at least one subscriber terminal transmits a network layer service request message, which contains the subscriber terminal identifier, on the dedicated channel to the network layer of the network part.
In the above-described manner the subscriber terminal requests a dedicated control channel on common control channels. Signalling needed for connection establishment is completed on the dedicated channel, whereafter a transfer to an actual traffic channel will be performed.
The network part cannot know when the subscriber terminal is ready to start communicating, therefore the subscriber terminal""s first message cannot be timed to avoid coincidence with the transmission of some other subscriber terminal.
The random access request message contains information on why the subscriber terminal wishes to establish the connection, and a random discriminator, but not an identifier to identify the subscriber terminal. In GSM system, for instance, the length of the random discriminator is only five bits. In UMTS (Universal Mobile Telephone System) system, the random discriminator is probably slightly longer.
Thus it is possible that at the same time instant two subscriber terminals transmit a random access request message with the same content. As result two things may occur in the network part: the first burst is received at a considerably higher power than the second and its content can thus be deoded, or, neither one can be received. The latter case is not problematic, a Slotted Aloha protocol is then used, according to which both subscriber terminals retransmit the random access request message in a random time.
The former case causes a problem which relates to the present invention. The network part transmits an access grant message, and two (or even more) subscriber terminals receive said message. If the data contained in the access grant message correspond to the content of the random access request message transmitted by the subscriber terminal, the subscriber terminal assumes having reserved a dedicated channel. This gives rise to a contention which subscriber terminal has succeeded in making the reservation. The problem is solved by a particular contention resolution procedure.
In GSM system the subscriber terminal transmits a data link layer SABM (Set Asynchronous Balanced Mode) frame containing a network layer service request message. The service request message contains the identifier of the subscriber terminal. The data link layer of the subscriber terminal stores the content of this frame to perform contention resolution. The network part returns the service request message in a data link layer UA (Unnumbered Acknowledgment) frame. The data link layer of the subscriber terminal compares the content of the UA frame information field, i.e. of the service request message, with the content of the stored message. If the contents are not identical, the subscriber terminal comes to a conclusion that its reservation failed. Then the subscriber terminal terminates its traffic on said dedicated channel and eventually restarts a new reservation operation for a dedicated channel. If the contents are identical, the subscriber terminal comes to a conclusion that the reservation succeeded and continues traffic on said dedicated channel.
The above-described arrangement can be used in GSM system. In new systems that are developed further from the basis of GSM system, for instance in UMTS (Universal Mobile Telephone System), the use of the described arrangement creates problems. When transferring protocols of the described GSM sublayers to UMTS, the contention resolution would be performed in an LLC (Logical Link Control) layer located in the data link layer to be described in greater detail later on in the text. The LLC sublayer can be used in the minimum mode to avoid peer-to-peer signalling between the network part and the subscriber terminal. Then the described arrangement does not work. Another problem with the known solution is that unnecessary data has to be transmitted over the air interface when the whole content of the network layer service request message is repeated in the UA frame, even though the mere identifier of the subscriber terminal would be sufficient.
The object of the invention is to provide a method and an equipment for implementing the method to the effect that the above problems can be solved. This is achieved by a method of the type described in the preamble, which is characterized in that a radio network sublayer included in the network layer of the network part transmits on a dedicated channel a response message containing a subscriber terminal identifier to a radio network sublayer of the subscriber terminal; the radio network sublayer of at least one subscriber terminal compares the identifier received in the response message on the dedicated channel with its own identifier, and if the identifiers are identical, it comes to a conclusion that its dedicated channel reservation succeeded, and if the identifiers are different, that its dedicated channel reservation failed.
The invention further relates to a network part in a cellular radio network comprising protocol software which is arranged to capture a random access request message transmitted on the random access channel by the subscriber terminal, to transmit an access grant message on the access grant channel to the subscriber terminal, to receive a service request message containing the subscriber terminal identifier transmitted on the dedicated channel by at least one subscriber terminal network layer.
In accordance with the invention, the network part of the cellular radio network is characterized in that the protocol software is arranged to transmit a response message, which contains the subscriber terminal identifier, of the radio network sublayer included in the network layer on the-dedicated channel to the radio network sublayer.
The invention further relates to a subscriber terminal in a cellular radio network comprising protocol software, which is arranged to transmit a random access request message on a random access channel to a network part, to receive an access grant message on an access grant channel transmitted by the network part and to assume on the basis thereof that a dedicated channel is reserved, to transmit a network layer service request message containing a subscriber terminal identifier on the dedicated channel to the network layer of the network part.
In accordance with the invention, the subscriber terminal in the cellular radio network is characterized in that the protocol software is arranged to receive a response message containing the subscriber terminal identifier transmitted on the dedicated channel by the radio network sublayer included in the network part""s network layer, to compare the subscriber terminal identifier in the response message received in the radio network sublayer with its own identifier, and to come to the conclusion that if the identifiers are identical, the reservation succeeded, and if the identifiers are different, the reservation failed.
The preferred embodiments of the invention are disclosed in dependent claims.
The invention is based on shifting the responsibility of the contention resolution procedure from the data link layer to the radio network sublayer of the network layer.
Several advantages are achieved with the method and arrangement of the invention. The solution enables current solutions in GSM to be flexibly transferred into UMTS. The structure of the response message is also made more sensible, whereby unnecessary data will not be repeated needlessly. Alternatively, the normal network layer message is used as a response message, whereby the transmission of an extra contention resolution message is avoided altogether.